This invention relates generally to high power laser sources and, more particularly, to high power laser sources providing output at visible or ultraviolet wavelengths. These laser sources are needed for a variety of applications, both military and commercial, but prior art sources have wavelengths restricted by the gain bandwidth of rare earth dopants used in fiber amplifiers cores.
In a typical prior art approach, output from a master oscillator (MO) is distributed to an array of high power fiber amplifiers pumped by laser diode arrays. The output beams from the fiber amplifiers are combined in a closely packed lens array to form the output beam. A sample of the output beam from each fiber is compared, on a detector array, to a frequency shifted reference wavefront derived from the MO, to provide a measurement of the instantaneous phase of each fiber amplifier in the array, and the phases are then corrected in real time to form the output beam. The output from the MO defines the spectrum and modulation waveform input to the amplifiers. A critical limitation is that the wavelength of operation is restricted by the gain bandwidth of the rare earth dopant used in the core of the fiber amplifiers. For the most efficient designs this wavelength happens to fall in the region of 1000 nm to 1100 nm using ytterbium (Yb) as the dopant. Unfortunately, this spectral region is not one that is always needed for high power laser applications, some of which call for output in the visible or the ultraviolet region of the spectrum.
The basic architecture of which the present invention is an improvement, is described in various prior patents, notably U.S. Pat. No. 5,694,408 to Bott et al., “Fiber Optic Laser System and Associated Lasing Method.” The present invention also utilizes a prior art technique for beam formation and phase control, as described in four other patents: U.S. Pat. No. 6,147,755 to Heflinger et al., “Dynamic Optical Phase State Detector,” U.S. Pat. No. 6,229,616 to Brosnan et al., “Heterodyne Wavefront Sensor,” U.S. Pat. No. 6,243,168 to Heflinger et al., “Dynamic Optical Micrometer,” and U.S. Pat. No. 6,366,356 to Brosnan et al., “High Average Power Fiber Laser System with High-Speed, Parallel Wavefront Sensor.”
As discussed in more detail below, the present invention employs harmonic generation properties of nonlinear crystals. These properties are generally known in the art, as described, for example, in U.S. Pat. No. 5,111,468 to Kozlovsky et al., “Diode Laser Frequency Doubling Using Nonlinear Crystal Resonator with Electronic Resonance Locking,” and U.S. Pat. No. 6,417,954 to Reynolds et al., “Nonlinear Optical Borates Materials Useful for Optical Frequency Conversion.” To the extent needed to provide a complete disclosure, the patents listed in this paragraph and the immediately preceding paragraph are incorporated by reference into this document.
It will be appreciated from the foregoing, that there is a need for a laser source that is both scalable to high powers and is operable at a selected shorter wavelength that is not restricted by the properties of dopants used in fiber amplifiers. The present invention satisfies this need and provides other advantages over the prior art.